Schottky barrier diodes with anode diameters in the submicron to micron range are generally known. Such devices are typically used as resistive mixer elements and varactor multipliers in the 100 GHz to 3 THz frequency range. The requirements of low shunt capacitance across the diode junction and low junction capacitance itself, however, severely restrict diode design. One diode in an array of Schottky diodes, which is contacted with a fine pointed wire or whisker having a diameter of from between 12 to 25 microns, has long been used to satisfy the need for minimum shunt capacitance. In order to operate at these frequencies, the pointed wire or whisker also provides a tuning inductance which can be varied by changing whisker shape and length. Shunt capacitance from the whisker is on the order of 1 fF (femto-farads), i.e. 1.times.10.sup.-15 farads, and zero bias junction capacitance values as low as 0.6 fF have been achieved with this type of device.
Whisker contacted diodes, however, have a number of serious disadvantages. For example, they are often mechanically unstable in vibration environments such as in a satellite launch and, due to the fact that operation is sometimes required at cryogenic temperatures in order to obtain desirable signal to noise ratios and enhanced performance, special mounting structures are needed to maintain whisker contact as the various elements change dimension on cooling. Also, whisker contacted diodes are virtually impossible to incorporate into integrated circuitry including diode arrays or integral antenna schemes. Furthermore, small area diodes having a diameter on the order of 1 micron or less require great skill and effort to contact.
As a result of these limitations, whiskerless Schottky diodes have been developed. Such diodes comprise epilayered structures and provide a significant improvement in eliminating some or all of the aforementioned problems. These devices typically are comprised of one or more layers of gallium arsenide formed on a semi-insulating substrate with an overlying anode contact pad and ohmic contact pad which may be adjacently located on the same or top side of the diode structure. One major problem still remains, however, and comprises the shunt capacitance from the anode contact pad to the underlying gallium arsenide layer. This capacitance is especially detrimental since it is orders of magnitude larger than the diode junction capacitance and other shunt capacitance components and since it provides a path for displacement current to flow at radio frequencies (RF) from the anode contact pad to the underlying conductive gallium arsenide layers. This current then flows to the ohmic contact through said conductive layers, effectively shunting the RF signal to be detected around the diode junction. This capacitance can be only partially eliminated by standard state of the art beam lead technology.
Several schemes have been resorted to recently in order to reduce or attempt to eliminate this major source of shunt capacitance. One of these comprises a mesa structure wherein the active gallium arsenide layers are etched away and the anode contact pad is formed over the material of the semi-insulating substrate. This technique is difficult to implement, however, because of processing problems associated with the formation and etching of oxide, photoresist and metal layers on a non-planar surface. These problems are further exacerbated by the need to produce very small anode diameters for high frequency operation.
Another technique involves a structure wherein a proton beam is used to convert the gallium arsenide material beneath the anode contact pad to semi-insulating material, thus eliminating the aforementioned processing difficulties. The disadvantage of this type of device is that there is a requirement for a relatively high energy proton source for conversion of relatively thick (3 to 4 microns), degenerately doped buffer layers of gallium arsenide and the additional photolithographic, oxide and metallization steps needed to protect the anode and ohmic contact areas from the proton beam.